Machinable glass-ceramics

ABSTRACT

Micaceous glass-ceramics are useful in the fabrication of single and multi-unit dental restorations including but not limited to orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutments, cylinders, and connectors by machining the glass-ceramic using CAM/CAM devices. The micaceous glass-ceramics are provided in a plurality of shades and colors to adequately match the colors and shades of teeth found in 95% or more of the human population.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/527,196 filed Mar. 17, 2000 now U.S. Pat. No. 6,375,729, whichapplication claims priority to Provisional Application Serial No.60/125,506 filed on Mar. 19, 1999, both which are hereby incorporated byreference.

FIELD OF THE INVENTION

This invention relates generally to a glass powder, which iscrystallizable and forms a sintered micaceous glass-ceramic in aplurality of shades and more specifically to micaceous glass ceramics,which are machinable into various dental articles by conventional tools.This material is especially useful for the fabrication of dentalrestorations using computer assisted design/computer assisted milling(CAD/CAM) devices.

BACKGROUND OF THE INVENTION

Micaceous glass-ceramic materials (i.e. glass-ceramics comprising acrystalline phase that belongs to the mica family such as tetrasilicfluormicas or fluorophlogopite micas) are known to exhibit excellentmachinability. However, their use as CAD/CAM materials for dentistry islimited by the inability to produce the required range of shades andtranslucency paramount for esthetically sound restorations. Thisseverely inhibits widespread use of micaceous materials as dentalrestoratives considering that the driving force for all-ceramicrestorations is esthetics superior to that of porcelain fused-to-metal(PFM) restorations. For example, Dicor MGC, available from DentsplyInternational Inc., Caulk Division, (located in Milford, Del.) is acommercially available micaceous dental ceramic for use in CAD/CAMdevices, but it is supplied in only two modifications, Dicor MGC—Lightand Dicor MGC—Dark. Other limitations of micaceous glass-ceramicsinclude high solubility and low strength in comparison to other dentalceramics. One such example is ProGlass™ ceramic available from CAD/CAMVentures LLC, (located in Irving, Tex.), which is a sugary-whitemica-containing material exhibiting a flexure strength of about 100 toabout 150 MPa and a solubility of about 1 mg/cm² (1000 μM/cm²).

At the same time, micaceous glass-ceramics exhibit far superiormachinability compared to other CAD/CAM ceramics such as sanidine-basedVita Mark II, available from Vita Zahnfabrik (Germany) and leucite-basedPro-Cad from Ivoclar (Lichtenstein), as set forth in “MechanicalProperties of a New Mica-Based Machinable Glass Ceramic For CAD/CAMRestorations” by J. Y. Thompson et al., The Journal of ProstheticDentistry, 1996, Vol. 76, No. 6, 619-623 and “Machinable Glass-CeramicsBased on Tetrasilicic Mica” by D. G. Grossman, Journal of Am.Cer.Soc.,1972, Vol. 55, No. 9. The latter two above-mentioned ceramics can bemachined by diamond tools only and require wet processing in contrast tomicaceous glass-ceramics such as ProGlass™ which can be machined bycarbide tooling using dry processing which is much more cost-effective.In addition, micaceous glass-ceramics can be much more translucent thanvery opaceous sanidine glass-ceramics.

U.S. Pat. Nos. 4,652,312, 4,431,420 and 5,246,889 are each directed tomica-containing ceramics that are formed from glass compositions and areshaped as glass and converted into micaceous glass-ceramics byconventional volume crystallization techniques. Each process involvesmelting glass batches, casting the glass melts into molds, andcrystallizing the glass into micaceous glass-ceramics. There is nodiscussion providing how to achieve adequate colors and shades toaccurately match the color of a person's tooth or how to control theshading of mica containing glass-ceramics. Any mention of colorantsappears to be directed to adding the colorants to the glass batch priorto melting. Such process does not effectively control the color of theresulting glass-ceramic. Furthermore, each of the processes appears toeffect crystallization by performing bulk or volume crystallization. Itis difficult to control the color of the micaceous glass-ceramics whenutilizing volume crystallization. None of the prior art is concernedwith the need to provide a variety of colors and shades to adequatelymatch the color and shade of a patient's teeth.

It is desirable to provide a variety of shades of micaceousglass-ceramics in order to fabricate restorations that closely andaccurately match the teeth in a patient's mouth. It is preferable toprovide an efficient and effective method of producing a variety ofshades of micaceous glass-ceramics. It is beneficial to providemicaceous glass-ceramics that are machinable and that come in a varietyof shades.

SUMMARY OF THE INVENTION

These and other objects and advantages are accomplished herein by themicaceous glass-ceramics comprising silica, magnesium oxide and fluorinein addition to other components listed below. The glass-ceramics areuseful in the fabrication of single and multi-unit dental restorationsincluding but not limited to orthodontic appliances, bridges, spacemaintainers, tooth replacement appliances, splints, crowns, partialcrowns, dentures, posts, teeth, jackets, inlays, onlays, facing,veneers, facets, implants, abutments, cylinders, and connectors bymachining the glass-ceramic using CAM/CAM devices. The micaceousglass-ceramics are provided in a plurality of shades and colors toadequately match the colors and shades of teeth found in 95% or more ofthe human population.

In accordance with one embodiment directed to the process of making theglass-ceramics, the batch ingredients of the compositions are melted ata temperature in the range of about 1200° to about 1650° C., for a timein the range of about 0.5 to about 8 hours, thereafter it is quenched,and pulverized into powder. Pigments, opacifiers, fluorescing agents andthe like are mixed with the powder. The powder is then used to formnet-shaped or block-shaped pre-forms or blanks to be used in CAD/CAMdevices. Blanks are dry-pressed and sintered using a one- or two-stepheating cycle at a temperature in the range of about 600° to about 1200°C. and for a time in the range of about 0.5 to about 4 hours for eachstep in the cycle. The sintering is preferably conducted in a vacuum.Occurring simultaneously with sintering, surface crystallization of thestarting glass powder yields the amount of mica phase of at least thirtyvolume percent (30 vol. %) required for machinability as well asstrength.

DETAILED DESCRIPTION OF THE INVENTION

As will be appreciated, the present invention provides glass-ceramiccompositions comprising a glassy matrix and one or more micaceous phases(e.g., tetrasilic flourmica, fluorophlogopite mica and the like). Theglass-ceramics are useful in the fabrication of dental restorations. Themicaceous glass-ceramic compositions contain inter alia, silica,magnesium oxide and fluorine in the ranges given in Table 1 below. Theglass-ceramic compositions have a combination of properties includinghigh strength and chemical durability useful for dental restorations.The glass-ceramics have good machinability, i.e., the ability to be cutor milled by a cutting tool into a dental restorative shape thataccurately depicts the original shape of the tooth to be restored orreplaced.

In an important aspect herein, the micaceous glass-ceramics are providedin a plurality of shades and colors to adequately match the colors andshades of teeth found in 95% or more of the human population. The shadesand colors of the glass-ceramics provide the dental technician with theability to closely and effectively match the color and shade of thepatient's tooth or teeth abutting or adjacent to the tooth or teeth thatis/are being restored or replaced.

TABLE 1 Compositions of the starting glass powder Wt % Wt % Oxide Mole %Range 1 Range 2 SiO₂ 30-65 43-72 43-72 Al₂O₃ 0-7  3-14 0-3 B₂O₃ 0-3 0-30-3 ZnO 0-3 0-3 0-3 CaO 0-5 0-7 0-3 MgO 15-33 10-30 10-30 TiO₂ 0-3 0-30-3 BaO + SrO 0-3 0-5 0-5 Li₂O 0-3 0-3 0-3 K₂O  0-10 0-7  7-19 Na₂O 0-70-3 0-3 CeO₂ + La₂O₃ ₊ 0-1 0-2 0-2 Tb₄O₇ ZrO2 0-4  0-10  0-10 F 14-25 5-10  5-10

In accordance with one embodiment of the method of the invention, theshaded micaceous glass-ceramics are manufactured by admixing pigmentsand other additives to the starting glass powder. The powder is formedinto pre-forms or blanks and the blanks are concurrently sintered andcrystallized. The resulting shaded blanks of various shades andtranslucency levels are consistent with current all-ceramic orporcelain-fused-to-metal (PFM) dental porcelain systems. The pre-formsor blanks may be machined into a dental restoration using a CAD/CAMdevice.

In accordance with the process, the glass compositions within the rangesgiven in Table 1 are melted at a temperature in the range of about 1200°to about 1650° C. and for a time in the range of about 0.5 to about 8hours, thereafter quenched, and pulverized into powder. This powder issieved to obtain the required particle size and mixed with conventionaladditives such as pigments, opacifiers, and fluorescing agents, whichwill produce various colors, shades and translucency levels aftersintering and concurrent crystallization have been performed. The powderthat contains the additives is then used to form net-shaped orblock-shaped pre-forms or blanks to be used in CAD/CAM devices. Theblanks are dry-pressed and sintered using a one- or two-step heatingcycle at a temperature in the range of about 600° to about 1200° C. andfor a time in the range of about 0.5 to about 4 hours for each step inthe cycle. The sintering is preferably conducted in a vacuum atmosphere.Occurring simultaneously with sintering, surface crystallization of thestarting glass powder yields an amount of mica phase of at least thirtyvolume percent (30 vol %) required for machinability as well asstrength. Copending commonly assigned U.S. application Ser. No.09/458,919, filed on Dec. 10, 1999, and U.S. Pat. No. 5,968,856 toSchweiger discuss volume crystallization and surface crystallization oflithium disilicate glass-ceramics and are hereby incorporated byreference.

In mass-production of CAD/CAM blanks, uniaxial pressing or other formingtechniques are utilized, e.g. CIP/HIP route whereby green bodies areformed in a CIP (Cold Isostatic Press) and subsequently sintered underpressure in an HIP (Hot Isostatic Press). As an alternative to the CIPmethod, the powder can be mixed with binder and pelletized or extruded.Useful CAD/CAM devices include the CEREC™ machine (available fromSiemens AG), the PROCAM™ machine (available from CAD CAM Ventures LLC inIrving, Tex. ) or copy-milling devices such as the Celay™ machine(available from Mikrona Technologie AG).

Essential for the present invention is an F content in excess of about14 mole percent (about 5 weight percent). Besides being a constituent offluormicas, F facilitates surface crystallization. Other ingredientsthat favor surface crystallization are B₂O₃, P₂O₅, BaO and Li₂O.

Essential for the present invention is the volume fraction, size andaspect ratio of the mica phase in the resultant sintered glass-ceramic.The larger the mica plates and the higher their aspect ratio, the lowerthe volume fraction of mica that is required to attain machinability. Anaspect ratio, i.e., ratio of thickness to length, of the mica plate of≧2 is preferred. At least thirty volume percent (30 vol. %) of mica isrequired to attain machinability. At least thirty volume percent (30vol. %) of the residual glass phase is required for sinterability.Therefore, the mica content is between about 30 and about 70 volumepercent, and preferably between about about 40 and about 60 volumepercent.

Some of the compositions of the present invention are extremely reactiveand will dissolve additives (e.g., pigments and fluorescing agents)during sintering of the CAD/CAM blocks. In this case, it was found thatusing coarser additives (e.g., pigments or fluorescing agents)substantially alleviated this problem. Normally, these additives(pigments and fluorescing agents) have an average particle size of about4 to about 8 microns. For most reactive compositions, it was found to becritical to use additives of average particles size equal to orexceeding 15 microns. It is believed that, at the same time, particlesexceeding 60 microns will compromise strength. Thus, special care shouldbe taken to remove particles greater than 60 microns. Preferably, theaverage size of the additives is in the range of about 15 to about 35microns, and more preferably in the range of about 20 to about 30microns.

Application of pressure during sintering of blanks such as in theCIP/HIP route described above is more expensive than vacuum sintering ofuniaxially pressed blanks but may be extremely beneficial in some cases.For example, the CIP/HIP route allows consolidation of powders havinghigh pigment load, or powders yielding high mica content uponcrystallization, or powders having high ZrO₂ content.

Extremely beneficial, especially for larger high aspect ratio mica, ispost-machining heat treatment of dental restorations at temperaturesbetween the glass transition temperature (GTT) and the dilatometricsoftening point (DSP). This heat-treatment affects surface crack healingand increases structural integrity of the restorations.

In addition to fluormicas, other phases can be present such ascordierite, apatite, spodumene and zirconia. Consequently the expansionof the resulting glass-ceramic can be varied in the range of about 7 toabout 12×10⁻⁶/° C.

A coating is preferably applied over the core material manufactured fromthe micaceous glass-ceramic to provide an aesthetically pleasingsurface. A suitable coating is a ceramic, glass-ceramic, a glass, aglaze and/or a composite material. It is advantageous that the coatinghas a coefficient of thermal expansion slightly less than the thermalexpansion of the core material. The coating is typically applied bysintering the ceramic, glass-ceramic, glass, glaze a composite materialonto the micaceous glass-ceramic core.

The following examples illustrate the invention.

EXAMPLE 1

A starting glass composition corresponding to the composition set forthin Table 2 below was batched from conventional raw ingredients andmelted at 1400° C. for 4 hours in a coarse-grained alumina crucible. Theglass melt was quenched into water. The quenched glass was dried andmilled into powder. The powder was screened to −200 mesh. Commercialpigments and fluorescing agents (yellow from Cerdec Co. (Washington,Pa.), pink from Engelhard Corp. (Iselin, N.J.) and jet black fromStandard Ceramic Supply Co. (Carnegie, Pa.)) were added to and blendedwith the powder. The powder was dry-pressed into 18×18×25 mm³ blocks.The blocks were fired in a vacuum of 20 torr using a two-step heatingcycle at 10° C./min to 650° C. and held for 2 hours at this temperatureand 10° C./min to 1100° C. and held for 4 hours at this temperature. Theblocks were sectioned into bars for three-point bend strength tests andsmall squares for solubility measurements according to ISO 9693solubility testing standards. The measurements are listed in Table 2below.

EXAMPLE 2

A starting glass composition corresponding to composition set forth inTable 2 below was batched from conventional raw ingredients and meltedat 1400° C. for 4 hours in a coarse-grained alumina crucible. The glassmelt was quenched into water. The quenched glass was dried and milledinto powder. The powder was screened to −200 mesh. Commercial pigmentsand fluorescing agents were used in concentrations given in Table 3below. Since it is known that all dental shades can be produced byvarying combinations of three basic pigments. i.e., yellow, red (orpink) and blue (or gray or black), the glass-ceramic of this example wasshaded using both individual yellow, pink and black pigments and theircombinations. Yellow pigments from Cerdec (Washington, Pa.), pinkpigments from Engelhard Corp. (Iselin, N.J.) and jet black pigments fromStandard Ceramic Supply Co. (Carnegie, Pa.) were used. The powderformulations were pressed into 2.5 gram disks and fired in a vacuum of20 torr using a two-step heating cycle at 10° C./min to 650° C. and heldfor 2 hours at this temperature and 10° C./min to 1100° C. and held for4 hours at this temperature. The resulting shades were evaluated usingthe ColorTec-SCM™ color computer from ColorTec Corp. (Clinton, N.J.) andwere found appropriate for simulating the color and shade of a person'steeth.

EXAMPLE 3

Blocks of lightly shaded glass-ceramics of Example 2 were sent to CADCAM Ventures LLC (Irving, Tex.) and to DentalMatic Technologies Inc.(Sainte-Laurent, Quebec, Canada) to be evaluated for machinability. Theywere machined into shapes roughly approaching that of a dental coping.The machinability was evaluated as satisfactory.

TABLE 2 Compositions of glass-ceramics in Examples 1 and 2 Wt % Wt %Oxide Example 1 Example 2 SiO₂ 40.8 61.0 Al₂O₃ 11.6 0.5 B₂O₃ 0 0 ZnO 0 0CaO 5.5 0 MgO 27.4 17.2 TiO₂ 0 0 BaO 0 0 Li₂O 0 0 K₂O 1.5 13.1 Na₂O 0 0CeO₂ 0 0 ZrO2 8.3 5 F 8.6 5.6 3-Pt bend 196 ± 24 >100 strength (MPa) Iso9693 8 <100 Solubility μg/cm² CTE 8° × 10⁻⁶/° C.) Not (25°-500° C.)Measured

TABLE 3 Example 2 powder with the addition of pigments. Glass powder ofexample Yellow Pink Jet Black CIE L*a*b* 2 41720 D320 K-60 Light Source:Wt % Wt % Wt % Wt % D65 - 10° 99.2687 0.7313 L* 53.31 a* −2.47 b* 21.33C* 21.47 h* 96.62 99.6693 0.3307 L* 55.461 a* 2.10 b* 0.22 C* 2.11 h*6.06 99.995 0.005 L* 54.67 a* −0.31 b* 0.08 C* 0.32 h* 164.84 98.6 1.499.3 0.7 99.99 0.01 98.933 0.7313 0.3307 0.005 98.59 0.7 0.7 0.01

The glass-ceramics of the invention have the capability to provide awide selection of shades and colors for matching the shades and colorsof a person's teeth. The glass-ceramics are readily machinable andprovide high strength and chemical durability to the dental restorationsmade therefrom.

While various descriptions of the present invention are described above,it should be understood that the various features can be used singly orin any combination thereof. Therefore, this invention is not to belimited to only the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

What is claimed is:
 1. A micaceous glass-ceramic for use in thefabrication of a dental restoration comprising a color and shadeselected from a variety of colors and shades to adequately match thecolors and shades of teeth found in 95% or more of the human populationcomprising: about 43 to about 72 weight % SiO₂; about 3 to about 14weight % Al₂O₃; about 10 to about 30 weight % MgO; about 5 to about 10weight % F; up to about 3 weight % B₂O₃; up to about 3 weight % ZnO; upto about 7 weight % CaO; up to about 5 weight % BaO and SrO; up to about3 weight % Li₂O; up to about 7 weight % K₂O; up to about 3 weight %Na₂O; up to about 2 weight % CeO₂, La₂O₃ and Tb₄O₇; up to about 3 weight% TiO₂; and up to about 10 weight % ZrO₂.
 2. A micaceaous glass-ceramicfor use in the fabrication of a dental restoration comprising a colorand shade selected from a variety of colors and shades to adequatelymatch the colors and shades of teeth found in 95% or more of the humanpopulation comprising: about 43 to about 72 weight % SiO₂; about 3 toabout 14 weight % Al₂O₃; about 10 to about 30 weight % MgO; about 5 toabout 10 weight % F; up to about 3 weight % B₂O₃; up to about 3 weight %ZnO; up to about 7 weight % CaO; up to about 5 weight % BaO and SrO; upto about 3 weight % Li₂O; up to about 7 weight % K₂O; up to about 3weight % Na₂O; up to about 2 weight % CeO₂, La₂O₃ and Tb₄O₇; up to about3 weight % TiO₂; and up to about 10 weight % ZrO₂; wherein the dentalrestoration is selected from the group consisting of orthodonticappliance, bridge, space maintainer, tooth replacement appliance,splint, crown, partial crown, denture, post, tooth, jacket, inlay,onlay, facing, veneer, facet, implant, abutment, cylinder, andconnector.